Cholesterol is a soft, waxy substance that is present in the bloodstream and in all cells. It is used to help digest fats, to help form cell membranes, and is an important precursor of many hormones. Although cholesterol plays an essential role in many key processes in the body, too much cholesterol in the blood (called hypercholesterolemia) can be dangerous: high cholesterol levels can lead, e.g., to the accumulation of cholesterol on artery walls, thereby elevating the risk of a number of cardiovascular disorders, including blood clots, heart attacks and stroke. In addition, one factor in the development of atherosclerosis is the formation of foam cells, i.e., macrophages or smooth muscle cells that contain high amounts of lipids such as cholesterol.
In animals, cholesterol is obtained from both external and internal sources. All animals can synthesize cholesterol de novo, which occurs mostly in the liver. In addition, a large amount of cholesterol is obtained from animal-based sources of food.
An important cause of hypercholesterolemia in humans is the absorption of excess cholesterol in the diet. While some cholesterol can be removed from the body, through the liver, this removal is often insufficient to lower cholesterol levels in patients with hypercholesterolemia to safe levels.
Sitosterolemia is a rare, recessive, inherited lipid disorder characterized by a multitude of symptoms, including xanthomas, atherosclerosis, elevated sitosterol levels, myocardial infarction, arthritis, and chronic hemolytic anemia. Sitosterolemia has been associated with an increased absorption, and decreased elimination, of dietary cholesterol as well as plant sterols, such as sitosterol, which are normally not absorbed from the diet (see, e.g., Online Mendelian Inheritance in Man (OMIM) entry 210250). The gene underlying sitosterolemia has been mapped to genomic region 2p21 (Patel et al., J. Clin. Invest. 102:1041–1044 (1998)).
The ATP binding cassette (ABC) family of transporters represents a large number of evolutionarily related transmembrane proteins that are involved in the transport of a diversity of substrates, including ions, drugs, peptides, and lipids, and which have been found in a large number of prokaryotic and eukaryotic organisms (for review, see, Higgins, Ann. Rev. Cell. Biol. 8:67–113 (1992); Croop, Meth. Enzym. 292:101–165 (1998); see, also, http://www.med.rug.n1/mdl/humanabc.htm for a list of human ATP family members).
Certain ABC family members have been associated with the transport of cholesterol and other lipids. ABC1, which has been shown to transport cholesterol and other lipids, has been linked to Tangier disease (see, e.g., Hobbs et al., J. Clin. Invest. 104:1015–1017 (1999); Lawn et al., J. Clin. Invest. 104: R25–R31 (1999)). In addition, ABC8, also referred to as ABCG1, has been shown to be involved in the transport of cholesterol and other lipids. While ABC1 is known to act as a monomer, ABC8 is a “half site” family member, which are thought to require dimerization to function as a transporter.
Recently, liver X receptors (LXRs) have been identified as key components in cholesterol homeostasis. Two LXR proteins (αand β) are known to exist in mammals. The expression of LXRα is restricted, with the highest levels being found in the liver, and lower levels found in kidney, intestine, spleen, and adrenals (see Willy, et al., Genes Dev. 9(9):1033–45 (1995)). LXRβ is rather ubiquitous, being found in nearly all tissues examined. Recent studies on the LXRs indicate that they are activated by certain naturally occurring, oxidized derivatives of cholesterol, including 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol and 24,25(S)-epoxycholesterol (see Lehmann, et al., J. Biol. Chem. 272(6):3137–3140 (1997)).
Mice lacking the receptor LXRα (e.g., knockout or (−/−) mice) lose their ability to respond normally to increases in dietary cholesterol and are unable to tolerate any cholesterol in excess of that synthesized de novo. These results have established the essential role of LXRα in the regulation of cholesterol homeostasis.
Clearly, new approaches for reducing the absorption of dietary cholesterol, for maximizing the elimination of excess cholesterol from the liver, and for preventing the development of foam cells would have tremendous public health benefits. The present invention addresses these and other needs.